The long-term goal of our research program is to understand how optical and retinal factors constrain the quality of visual experience. In central vision, poor optical quality of the eye is the chief cause of poor visual performance on many visual tasks, especially in clinically abnormal eyes. We propose to use newly developed technologies to investigate the underlying physiological mechanisms which are responsible for these optical limitations to vision. The results will improve our understanding of the optical consequences of clinical conditions such as dry eye and keratoconus, which may ultimately lead to new diagnostic and treatment strategies. Aim 1 is to test the hypothesis that the corneal tear film provides an optically smooth refracting surface which reduces light scatter and refractive aberrations that would otherwise degrade the retinal image and reduce visual performance. This hypothesis predicts that disruption of the tear film by blink suppression will expose the underlying rough, irregular surface of the cornea, thereby increasing light scatter and refractive aberrations which will degrade the retinal image. The predicted outcome is blurry vision and reduced visual performance. To test these predictions we will use a Shack-Hartmann aberrometer to objectively measure refractive aberration and light scatter simultaneously at 200 or more points in the eye's pupil. The results will be compared with topographic maps of tear film disruption obtained simultaneously by fluorescein and by retro-illumination imaging of the pupil. Image quality will also be compared with visual acuity and contrast sensitivity during tear film disruption. Aim 2 is to test the hypothesis that corneal shape is responsible for the majority of the eye's refractive aberrations. In the process, we also aim to resolve the current controversy over whether optical aberrations due to corneal shape compensate or exacerbate the aberrations of the remaining optical elements of the eye. This hypothesis will be tested in normal eyes and in clinical patients with highly abnormal corneal shape caused by the corneal disease keratoconus. We will measure the aberrations of the cornea with topographic videokeratoscopy for comparison against aberrations of the whole eye measured with the Shack-Hartmann aberrometer. The effect of corneal aberrations and scatter in keratoconic eyes on visual performance will also be measured by using simulated retinal images computed with an optical model of the keratoconic eye as visual stimuli for normal eyes. Aim 3 is to develop a comprehensive, quantitative optical model of the eye which accounts for constraints on retinal image quality and on visual performance imposed by optical imperfections of the eye. Results from our studies of the optical function of the tear film and cornea will be incorporated in to the optical model to provide a quantitative, functional description of the effect of the eye's optical imperfections on retinal image quality and on visual performance in normals, dry-eye patients, and keratoconus patients.